What the manual and the first commenter are trying to say is that mb_strcut uses byte offsets, as opposed to mb_substr which uses character offsets.

Both mb_strcut and mb_substr appear to treat negative and out-of-range offsets and lengths in the basically the same way as substr. An exception is that if start is too large, an empty string will be returned rather than FALSE. Testing indicates that mb_strcut first works out start and end byte offsets, then moves each offset left to the nearest character boundary.